This invention relates to optical systems for monitoring the state of polarization (SOP) of an optical beam.
The transmission of information over optical fibers is pervasive in modern communication networks. Optical fibers are often favored over electrical cable because optical fiber offers much larger bandwidths than cable. Moreover, optical fiber can connect nodes over larger distances and transmit optical information between such nodes at the speed of light. Among factors limiting transmission rates and distances in high-speed fiber systems, however, are polarization effects such as polarization mode dispersion (PMD) in optical systems such as optical fibers.
Polarization mode dispersion arises from small, random birefringences in optical fibers. For sufficiently short sections of fiber, birefringence may be considered uniform, and light traveling along the fast and slow axes experience different propagation delays. For longer sections of fiber, however, the orientations and amplitudes of the birefringence varies, leading to a phenomenon called polarization mode coupling. The polarization mode coupling eventually randomizes the polarization state of the propagating optical signal. PMD also results in pulse broadening, which reduces the available bandwidth of the optical fiber.
It is therefore desirable to reduce the effects of PMD. This can be accomplished by compensating for PMD by detecting or analyzing the state of polarization (SOP) of the optical signal, and passing the optical signal through a polarization modulator (e.g., a variable retarder stack) to reduce such PMD effects in response to the detected state of polarization of the signal. For example, the polarization modulator can impart a retardance that is exactly opposite to that experienced by the optical signal in the fiber. An example of a prior art polarization compensation system is described by T. Chiba et al., in xe2x80x9cPolarization Stabilizer Using Liquid Crystal Rotatable Waveplates,xe2x80x9d in Journal of Lightwave Technology, Vol. 17, No. 5, May 1999.
Referring to FIG. 1, an example of a polarization analyzer 10, disclosed by Chiba et al., includes a pair of beamsplitters 12 and 13 position in the path of a beam 11 of unknown SOP. Beam splitters 12 and 13 direct sample beams 22 and 23 toward polarizing beamsplitters 30 and 32. Prior to contacting polarizing beamsplitter 32, sample beam 23 passes through a quarter wave plate 25. Polarizing beamsplitters 30 and 32 split sample beams 22 and 23 into orthogonal X and Y polarization components, where the X polarization component is the linear component in the plane of FIG. 1 and the Y polarization component is the linear component orthogonal to the plane of FIG. 1. The intensity of each component is measured by photodiodes 35-38. Quarter waveplate 25 is oriented at 45xc2x0 with respect to the nominal X and Y directions, therefore if dectectors 35 and 36 measure the relative linear x and y components of beam 11, then detectors 37 and 38 measure the right-hand and left hand circular components. Accordingly, the SOP of beam 11 can be determined from these two sets of orthogonal components.
The invention features an integrated optical assembly for providing information about the state of polarization (SOP) of an input beam passing through the assembly. Hereinafter, the assembly is also referred to as an SOP detector. The assembly includes multiple polarization-sensitive interfaces each providing a sample beam having an intensity providing information about the SOP of the input beam. For example, the assembly may provide four or more sample beams, the intensities of which are sufficient to uniquely determine the SOP of the input beam. Alternatively, the optical assembly may provide fewer than four sample beams, where the intensities of the sample are sufficient to determine the SOP of the input beam when combined with some a priori knowledge about the nominal SOP of the input beam. Furthermore, the optical assembly may provide multiple sample beams (e.g., 2 or 3 or more beams) whose intensities indicate a deviation of the SOP of the input beam from a desired SOP. The measured deviation can be used to provide a feed-forward or feed-back signal to a polarization modulator that alters the SOP of the beam.
At least two, and preferably all, of the polarization-sensitive interfaces in the optical assembly are oriented to direct the sample beams in a similar direction and to allow a compact integration of the optical assembly components. For example, the polarization-sensitive interfaces can be oriented substantially parallel to one another. Because the sample beams propagate in a similar direction, a single detector array can be used to monitor the intensities of the sample beams and add to the compactness of the overall optical system.
The optical assembly also includes a retardation layer positioned between each pair of similarly-oriented polarization-sensitive interfaces. The retardation layer(s) alter the polarization state of the beam to allow the polarization-sensitive interfaces to sample different polarization components of the input beam, and cause the intensity of each sample beam to provide different information about the SOP of the input beam. In preferred embodiments, such retardation layers are oriented similarly to the polarization-sensitive interfaces to improve the compactness of the optical assembly. For example, they can be oriented substantially parallel to the polarization-sensitive interfaces. Such a construction can be accomplished by forming each polarization-sensitive interface adjacent an optical window used to support a retardation layer. This can result in a monolithic and compact integration of the optical assembly components.
To separate the sample beams from the input beam, the optical assembly defines an optical beam path that contacts each of the polarization-sensitive interfaces at a non-normal angle (e.g., an angle of about 45xc2x0). In preferred embodiments, the optical assembly may further include an input prism and/or an output prism having a surface oriented substantially normal to the optical beam path to increase the coupling efficiency of the input beam into and out of the assembly. The assembly may also include a pre-compensation retarder to adjust the polarization state of the input beam prior to it contacting any of the polarization-sensitive interfaces and/or a post-compensation retarder to adjust the polarization state of the beam upon exiting the assembly. For example, the post-compensation retarder can be selected to cancel or minimize any change in the SOP of the input beam caused by passing through any of the intermediate retarders and/or polarization-sensitive interfaces.
The polarization-sensitive interfaces are constructed to sample only a small fraction of the input beam energy. For example, the optical assembly can have an insertion loss of less than 1 dB, or even less than 0.5 dB, or even less than 0.2 dB. Thus, the optical assembly can be positioned in the path of optical beam (e.g., a beam carrying optical telecommunication information) without significantly degrading the beam emerging from the assembly. Furthermore, the sample beams produced by the optical assembly have intensities that directly provide information about the SOP of the input beam. In other words, additional optical polarization processing of the sample beams is not necessary, thereby eliminating additional optical components and further adding to the compactness of the overall system.
The SOP detector can be used in a polarization control system that further includes a polarization modulator that adjusts the polarization of the input beam in response to a feed-back or feed-forward signal generated from the intensities of the sample beams. The polarization control system can be used to stabilize a varying SOP in an optical signal beam caused by effects such as PMD. Thus, the beam enters the polarization control system with an unknown (or only a nominally known) time-varying SOP and emerges from the system with a selected, well-defined SOP.
In general, in a first aspect, the invention features an integrated optical assembly including a series of polarization-sensitive interfaces defining an optical beam path for an input optical beam to pass through the assembly, wherein each polarization-sensitive interface derives a sample beam from the input beam. The integrated optical assembly also includes one or more retardation layers each positioned between a different pair of the polarization-sensitive interfaces, wherein the retardation layers are integrally coupled with the polarization-sensitive interfaces, and wherein the retardation layers and polarization-sensitive interfaces cause each sample beam to have an intensity that provides different information about the state of polarization of the input beam.
Implementations of the integrated optical assembly can include one or more of the following.
One of the retardation layers can be oriented substantially parallel with one of the polarization-sensitive interfaces. Each polarization-sensitive interface can derive less than 5% (e.g., less than 2%) of the input beam intensity to produce the corresponding sample beam.
At least two of the polarization-sensitive interfaces can be oriented substantially parallel to one another. For example, all of the polarization-sensitive interfaces can be oriented substantially parallel to one another.
The optical beam path can contact each polarization-sensitive interface at a non-normal angle (e.g., in the range of 30 degrees to 60 degrees). The series of polarization-sensitive interfaces can include three polarization-sensitive interfaces providing three sample beams. One or more retardation layers can include two retardation layers and the three polarization-sensitive interfaces can alternate in position with the two retardation layers. Moreover, the polarization-sensitive interfaces and the retardation layers can be oriented substantially parallel to one another. One of the two retardation layers can define a half-wave retardance with respect to the optical beam path and the input beam wavelength., and the other of the two retardation layers can defines quarter-wave retardance with respect to the optical beam path and the input beam wavelength. The input beam wavelength can be in the range of 1.2 microns to 1.7 microns.
The retardation layers can have fast axes oriented perpendicular to the optical beam path.
Each polarization-sensitive interfaces can preferentially reflect S-polarized incident light to produce the corresponding sample beam. The polarization-sensitive interfaces can include four polarization-sensitive interfaces providing four sample beams, and the intensities of the four sample beams can be sufficient to uniquely determine the state of polarization of the input beam.
The integrated optical assembly can also include an input prism positioned prior to the first polarization-sensitive interface with respect to the optical beam path. The input prism can have a first surface positioned to receive the input beam at substantially normal incidence and a second surface substantially parallel to the first polarization-sensitive interface. An input retardation layer can be included adjacent the first surface of the input prism.
The integrated optical assembly can further include an output prism positioned after the last polarization-sensitive interface with respect to the optical beam path. The output prism can have a first surface positioned substantially parallel to the last polarization-sensitive interface and a second surface substantially normal to the optical beam path. An output retardation layer can be included adjacent the second surface of the output prism.
The integrated optical assembly can also include a pair of transparent substrates having inner surfaces sandwiching each retardation layer. Each polarization-sensitive interfaces can be located at an outer surface of a corresponding one of the transparent substrates. One or more retardation layers can include two retardation layers and one of the polarization-sensitive interfaces can be a defined between the outer surfaces of adjacent ones of the transparent substrates for the two retardation layers. Another of the polarization-sensitive interfaces can be defined between the outer surface of the corresponding transparent substrate and a surface of an input prism. A third of the polarization-sensitive interfaces can be defined between the outer surface of the corresponding transparent substrate and a surface of an output prism. Each polarization-sensitive interface can include a coating on the outer surface of the corresponding transparent substrate. The coating can have an optical thickness along the optical beam path substantially equal to a quarter of the input beam wavelength. The coating can include a material having a refractive index lower than that of the transparent substrate (e.g., MgF2).
The thickness of the transparent substrates can be at least 0.4 mm (e.g., at least 1.0 mm), and can include glass layers.
In another aspect, the invention features a state of polarization detector including an integrated optical assembly as described above and a detector array positioned to receive the sample beams from the integrated optical assembly, and during operation the detector array measures the intensities of the sample beams.
The detector array can include a plurality of detector elements, each positioned to receive a sample beam from the integrated optical assembly.
In a further aspect, the invention features a polarization controller system, include a state of polarization detector as described above and a polarization compensator, which during operation adjusts the polarization of the input beam. The polarization controller also includes a controller, which during operation receives a signal from the state of polarization detector and adjusts the polarization compensator based on the information from the intensities of the sample beams.
Implementations of the polarization controller system can include one or more of the following.
The state of polarization detector and polarization compensator can be positioned to first determine and then to adjust the state of polarization of the input beam.
The state of polarization detector and polarization compensator can be positioned to first adjust the state of polarization of the input beam and then determine the state of polarization of the adjusted beam.
The polarization compensator can include a stack of at least three variable retarders.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the apparatus, methods, and examples are illustrative only and not intended to be limiting.
Additional features, objects, and advantages of the invention will be apparent from the following detailed description and drawings, and from the claims.